The present disclosure relates to a light-emitting diode, and in particular, to an AlGaInP-based light-emitting diode (LED) device, allowing red or infrared light-emitting diodes to have improved light extraction efficiency.
The present disclosure also relates to a structure of a transparent electrode and a method of designing the same.
Generally, an epitaxial layer with a small band gap is used in a red or infrared LED, and an amount of light to be absorbed by an electrode is large. Thus, the red or infrared LED suffers from low light extraction efficiency. Technologies of reducing a thickness of the epitaxial layer or increasing optical transmittance of the electrode have been used to overcome these problems.
The red or infrared LEDs is classified into two types: a p-side up structure and an n-side up structure. For the p-side up structure, when an epitaxial layer is grown on a gallium arsenide (GaAs) substrate, a surface of a p-type epitaxial layer is located at the topmost level of the device. Thus, if the GaAs substrate is used as a part of the LED device, most of light generated in the active layer may be absorbed by the GaAs substrate with a small band gap, and thus, the LED device suffers from low optical power.
To overcome this problem, the n-side up LED structure was introduced. For the n-side up LED structure, to reduce an amount of light to be absorbed by the GaAs substrate, a wet etching process is performed to remove the GaAs substrate from the p-side up LED structure and to allow an n-type epitaxial layer to be positioned at the topmost level of the device. In this structure, since the GaAs substrate is removed by the wet etching process and a total thickness of the epitaxial layer is reduced, it is possible to reduce an absorption amount of light generated in the active layer and thereby to increase optical power.
However, for the n-side up LED structure, a current spreading layer and a window layer (e.g., GaP, AlGaAs, and so forth) are not provided at the topmost level. This may lead to a reduced current spreading effect in a lateral direction and may set a limitation on improvement of the light extraction efficiency. To overcome this problem, a p-side up omni-directional reflector (ODR) LED structure was suggested. For the p-side up ODR LED structure, a current spreading layer and a window layer are provided at the topmost level of the LED device, and an ODR structure is provided on or in an n-type semiconductor layer. In addition, since the GaAs substrate is removed, it is possible to prevent light from being absorbed by the substrate and thereby to increase optical power of the LED device. A metal (e.g., gold (Au)-based) electrode is provided at the topmost level of the p-side up ODR LED structure. However, even in this structure, light generated in the active layer may be absorbed by a metal electrode.
An electrode of a conventional red LED device is formed of gold or a gold alloy-based material, regardless of whether it is of the p-side up structure or an n-side up structure. However, the gold alloy-based electrode has low reflectance (e.g., of 85-88%) within a red wavelength range (e.g., of 600 nm-700 nm), and thus, an amount of light to be absorbed by the electrode is increased to reduce an overall light efficiency of the LED device. In addition, during a thermal treatment process, an inter-diffusion or inter-mixing phenomenon may occur in the electrode, and this may lead to a reduction in reflectance of a reflection electrode and a reduction in light extraction efficiency of the LED device.
In some embodiments, an ohmic contact layer and a transparent electrode are provided at the topmost level of the p-side up ODR LED device to increase an overall optical power of the LED device.